Structural Control And Wear Resistance Of Al₂O₃-YAG Amorphous Coatings

The wear resistance of materials under high PV value conditions(P:contact pressure,V:friction rate)is the key factor to determine the service reliability and life of related mechanical component in mechanical systems such as aerospace and aeronautical power systems and aircraft transmission systems.And high PV value condition is usually accompanied with high temperature,oxygen enrichment,wide temperature range thermal shock and so on,which have higher requirements for the mechanical,thermal conductivity and fatigue failure resistance of the related material.Coating technology is a useful way to protect the above materials.The oxide ceramic coating has high hardness,antioxidation,high temperature resistance,microstructure stability and wear resistance to satisfy the above-mentioned service requirements,wherein the Al₂O₃ coating is a typical representative of the above conditions.However,the Al₂O₃ coating is prone to phase change in plasma spraying,which reducing coating thermal conductivity and mechanical performances.Meanwhile,the Al₂O₃ coating is easy to fatigue failure under the cyclic contact stress of friction and wear with high PV value,which reduces the service life of the coating.Therefore,a novel wear resistant coating need to be developed.In this study,Al₂O₃-YAG amorphous coating was prepared by atmospheric plasma spraying process combined with the synthesis of spraying powder.At the same time,the mechanism of as-sprayable powders,the amorphous formation mechanism,the crystallization kinetics behavior of amorphous coating and the friction and wear behavior of coating under severe service conditions with high PV value were systematically studied.The main result as following:(1)The Al₂O₃/YAG as-sprayable powders for preparing amorphous coating were successful manufactured.Al₂O₃ and Y₂O₃ powders were used as raw materials.According to the mechanism of high temperature solid phase reaction,Al₂O₃/YAG powders were synthesized by multiple step-by-step heating cyclic calcination.The main body of the coating deposited by this powder was amorphous,the structure of the coating was compact,and the porosity was 1.21±0.33%,the Vickers hardness was 6.61±0.12GPa.(2)The solid reaction mechanism of Al₂O₃-Y₂O₃ powders were explained,which mainly controlled by the chemical reaction rate at the phase interface and the ion diffusion rate in the solid phase.The initial reaction process was usually controlled by chemical reaction rate,then the reaction process will be transformed to ion diffusion rate with the temperature increasing.The diffusion model of?-Al₂O₃ was intracrystalline diffusion and grain boundary diffusion,while that of c-Y₂O₃ was intracrystalline diffusion.(3)Three elements of Al₂O₃-YAG amorphous coating prepared by thermal spraying were summarized.The preparation of Al₂O₃-YAG amorphous coating requires three key conditions:high enthalpy,ultra-fast cooling rate and the composition of spraying powder is low eutectic ratio.(4)The thermal crystallization mechanism of Al₂O₃-YAG amorphous coating was elucidated.The nucleation mechanisms for YAG and?-Al₂O₃ were nearly the same,both crystallization processes began to nucleate at a gradually decreasing nucleation rate,and the crystal growth process was gradually transformed from three-dimensional bulk growth to one-dimensional flake thickening.The difference was that YAG takes precedence in nucleation and growth,and the difficulty of breaking the energy barrier for YAG nucleation was similar with that for crystal growth.However,the energy barrier required for nucleation of?-Al₂O₃ was much smaller than that of crystal nucleus growth,and nucleation was extremely easy occurred at the critical temperature.(5)Al₂O₃-YAG amorphous coating was more suitable for high PV value condition.The Al2O3-YAG amorphous coating exhibits the ability to resist crack initiation and propagation under the friction and wear condition of higher PV value,and initially embodies the potential of resisting fatigue failure.(6)A kind of coating which is more suitable for higher PV value and has anti-fatigue failure property was obtained.Adhesion wear was the main wear process of Al₂O₃-YAG amorphous coating at high PV value condition.Meanwhile,the surface of the coating will undergo plastic deformation and crystallization under the thermal-mechanical coupling action,so the fatigue failure phenomenon under cyclic contact stress of Al₂O₃ coating or other brittle materials can be avoided.In addition,compared with the Al₂O₃ coating,the wear rate of both Al₂O₃-YAG amorphous coating and corresponding graphite disc decrease under the condition of higher PV value,so the Al₂O₃-YAG amorphous coating was expected to be applied under the condition of higher PV value.